If memory serves (I did this more than a year ago), I achieved this effect by placing sorters inline with the feeds to my storage tanks. You can't stop the drills from filling up, but you can use the sorters to stop the rock from passing through the to storage containers on your ship. PAM never sees the ship get full (or at your preset limit), so it keeps on drilling for as long as you have power.

I used a cheat small gravel reactor, placed a small stone to gravel converter in there, and viola! Never ending power because you have never ending fuel supply.

BTW, setting a connector to "dump" doesn't work, because that leaves blockage in your tunnel. My method "disappears" the rock.

Pro tip: Set your sorters to block stone, but not other valuable materials. Stone is everywhere, but these other resources should never be squandered (IMO).

Anyway, I gathered a bunch of useful materials while digging a 3 ship by 3 ship tunnel 200 meters into a mountain, then hollowing out the base itself deep inside. Once started, it ran by itself non-stop (except for the occasional drop of useful stuff) until complete.

I did spend a lot of time at the end of the tunnel manually digging up and down and side to side enough to give PAM enough room to maneuver so it could start digging out the base proper, but once that was done, the rest finished fully automated also.

Strongly recommend you use the "level" function of PAM before setting the job; only way to ensure you get a gravity-level floor. Failing to do this makes building the base problematic.

Enjoy.